Protecting a line circuit from excessive voltages and currents that can occur on the line to which it is connected is a well-known requirement. These excessive voltages and currents may be the result of relatively rare occurrences in the service lifetime of the line circuit, such as lightning strikes, or more common occurrences, such as short circuits to other lines or ground. Size, cost, and heat dissipation of the protection arrangement are important design considerations since a protection arrangement is needed for each subscriber line.
A protection arrangement for a line circuit is disclosed by Rosch et al. U.S. Pat. No. 4,947,427 issued Aug. 7, 1990 and entitled “Protection Arrangement for a Telephone Subscriber Line Interface Circuit”. While the protection arrangement appears to be suitable for over-current conditions of long duration, it might not be a desirable solution for ones of the short duration. Momentary short circuits, between the tip and ring lines of the subscriber loop, or between the ring line and ground, are often long enough in duration to cause an over-current condition. The protection arrangement of Rosch could be used to protect the line circuit during these short duration events. However, under such use, and if the events happened often, the isolation relay of Rosch could prematurely wear-out due to frequent opening and closing of the relay contacts.
A fault condition of short duration that interrupts the subscriber's telephone service, is likely to cause a loss of service that is much longer than the actual duration of the fault condition, especially if the line circuit must be manually returned to a normal operating condition after the fault condition has ended.
A further consideration is designing a protection arrangement for a line circuit is that there are different types of phone sets each having different requirements. For example, a plain old telephone set (POTS) telephone typically requires 18 mA to 90 mA of DC loop current, which is provided by a 52 volt battery in the central office. The line circuit typically has a 200 ohm feed resistor in each of the tip and the ring lines of the subscriber loop. The length of the subscriber loop, that is, the distance between the telephone and the central office, can result in a typical loop resistance (including the telephone set) in the range of 100 to 2000 ohms. For short loops having a resistance of less than 600 ohms, current limiting during normal operation is required to prevent the loop current from exceeding a maximum current limit. This maximum current limit can range from 30 to 50 mA, for example, depending on the telephone system. Excessive current beyond the maximum current limit could damage the telephone set, the feed resistors of the line circuit, or other components in the telephone system.
A current limiting arrangement is disclosed by Jakab in U.S. Pat. No. 5,333,196 issued Jul. 26, 1994 and entitled “Current Limiting Battery Feed Arrangement”. Although the arrangement appears to provide the desired current limiting result, the dollar cost, circuit board space and heat-dissipation of the components in the arrangement might outweigh the benefits derived from them in some applications. Additionally, some types of telephone sets have certain characteristics, which obviate the need for the type of current protection provided by the arrangement disclosed by Jakab. For example, in the case of a type of telephone set known as an electronic telephone set, the set acts as a current sink which controls the amount of current flowing in the subscriber loop. Thus, the need for current limiting in short loops having low loop resistance is eliminated under normal operating conditions. Current limiting under fault conditions, such as when the tip and ring lines are shorted together, or when the ring line is shorted to ground, is still a requirement for this type of set.
In view of the above, it appears that it would be desirable to have a voltage and current protection arrangement for a line circuit that takes advantage of the current limiting capability of electronic telephone sets under normal operating conditions.